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Help with Solar system sizing

I want to build a small solar system to power my FT-897D radio (22A at 100W transmit) and a few other small things. I am using 25A as my max. load. Just receiving, my load is probably 3A or less. Max. usage is about 3 hours per day – mostly listening. How many watts of solar panels do I need and how many Amp Hours of battery capacity do I need? What size controller and wire do I need?

Does anyone know how to calculate the answers to these questions? I have found quite a few solar sizing calculators on the internet, but the results they generate vary greatly. I would think there are some good rules of thumb for this – solar power for a 100w transceiver. Any suggestions?

I want to build a small solar system to power my FT-897D radio (22A at 100W transmit) and a few other small things. I am using 25A as my max. load. Just receiving, my load is probably 3A or less. Max. usage is about 3 hours per day – mostly listening. How many watts of solar panels do I need and how many Amp Hours of battery capacity do I need? What size controller and wire do I need?

Does anyone know how to calculate the answers to these questions? I have found quite a few solar sizing calculators on the internet, but the results they generate vary greatly. I would think there are some good rules of thumb for this – solar power for a 100w transceiver. Any suggestions?

73, Richard

First, the smart-ass answer: You really should ask this in the Astronomy Forum.

3A at 12v is 36 watts. 3 hr per day means about 100 watt hours per day. With a 3 day reserve, that's about a 300 usable watt hour battery. The 25A or 300 watt transmit pull is a consideration primarily as peak load but probably insignificant as far as energy storage in the context you describe.

300 watt hours implies about a 30# battery. That means that even a couple of the gel cells you find in UPS devices will do although a small car or RV battery would probably be cheaper.

A 100 watt hour solar harvest per day implies a solar panel of about 50 watts. You can find 30 cell solar panels of this size intended for direct connection to lead acid batteries as trickle chargers (30 cells peaks at 15v). More common is the 36 cell 18v panel and that would need a charge controller.

One of the biggest problems is going to be battery maintenance. Lead acid batteries need some activity to stay healthy. When not in use, they need a full charge that doesn't overdo it and they need a sulfation inhibiting technique applied. I am using a charge controller by BatteryMINDer(tm) that wasn't that expensive (~$90), handled 100 watts, and uses their pulse charge battery maintenance technique. I also have a 20A battery charger I can use now and then to help with the activity level.

As for wire size, 10g is usually good for up to 30 amps if the run is rather short. Look at the power cable that comes with your rig as an example.

The articles in the February QST on this topic were rather disappointing as they repeated some old myths and didn't really offer anything useful in practical terms. There is so much bad and misleading advice and information out there on this topic you do need to be careful. Read critically and think through the implications and context carefully.

If you're receiving 90% of the time (10% transmitting), in a 3 hour span you would consume roughly 128 watt hours in a typical day (receive is @ 1 amp, transmit is @ 22 amps on the FT-897D; 1 amp for 2.7 hours is 2.7 amps, 22 amps for 0.3 hours is 6.6 amps). With this in mind, I would recommend starting with a very basic solar setup.

For the battery, I would use something around a 20 to 30 amp hour sealed lead-acid battery (like this). To size a solar panel, you'll want to consider the average number of hours of sunlight you have available and the size of the battery (in watt hours). A 180 watt solar panel could charge up a 26 amp battery in roughly 2 hours. Something like this would work...

The last (very important) thing you'll need is a solar charge controller. It's job is to keep you from cooking your batteries once they're full. I definitely recommend the CirKits SCC3. It's a fun kit to build and it even adjusts the voltage based on temperature. Just set it to turn the green light on at 13.85 volts and you'll never cook the battery.

For a 24 amp load, be sure to use 8 AWG wiring (based on this chart) and don't forget the fuses (something like a RIGRunner 4005)!

There has to be at least 50 web sites dedicated to solar energy calculations, replete with plug in formulas—including average and peak loads, array size, Lat-Long adjustments, solar-days adjustments, and a few more so esoteric, even electrical engineers will be confused. Based on this, I suspect the vast majority of amateur radio operators mis-enter the requisite data. This includes your estimates—they're way low!

Just like you, I thought joining the Green Crowd might be a good thing. So, I did a bit of reading, and in the end, I was more confused than I was when I started. That was because I didn't understand the real parameters. So, I contacted someone I knew. He owns three commercial radio sites (cellphone, repeaters, etc.) which all rely on solar power. I got a real rude awakening! It turns out, that a decent system that one can rely on to provide a decent operating time (we are talking about emergencies too, aren't we?), cost many more dollars than most amateur operators are willing to spend. Digressing....

A solar powered station is a whole lot more than just a storage battery or two, a solar panel(s) of some sort, a regulator, wiring, fusing, mounting hardware, and a few miscellaneous parts. Just one or two of these devices is not enough!

With his help, I set about getting to the bottom line of what was needed if I used my IC-7600 for just 4 hours per day—a very low estimate in an emergency situation (I selected 12 hours without Sun which is about average for my Lat-Long). I wanted to make sure that the voltage supplied to the transceiver was within its IMD specs, which meant I'd needed a battery booster. And, although I live in Roswell, NM where the sun shines a lot in the Summer, that's not the case in the Fall, and Winter months. That obviously changed the formula. As I got deeper into the subject, things started to get really expensive!

The end result was, I needed nearly $4,000 worth of solar panels, $1,500 worth of batteries, an expensive regulator, wiring, battery house(ing), and a host of other hardware. Total cost? Just over $9,400! I therefore decided, that my mobile installation was much more economical, even with $4 per gallon gasoline!

Thank you to those who offered thoughtful answers. Like K0BG, the more I read about Solar PowerSystems the the more confused I get. Getting a generator would be easier and maybe cheaper. Nevertheless, thanks for the recommendations.

Does anyone know of any good websites, computer programs, books, etc. that might be of help to me? Aside from the "garbage in" problem, all this stuff should be pretty straight forward math - if you know how to do it. It should be pretty easy for a computer program to do it - I would think. Like K1CD, I found the February QST articles interesting, but disappointing in terms of useful information. The ARRL book, Emergency Power, is similar. It contains a lot of interesting information - but never really pulls it all together in a user friendly manner.

Richard, you are on your way and have gotten some good pointers. As you are finding out, there is quite a bit of detail and its not a small project. My post in the Tech Forum got some good pointers from others also but the most helpful things was finding someone local that has actually pulled something together. If you can find someone local, it helps. I was able to look at sites and information with my friend's help, which helped me make choices. The issue is really not your radio, whose power draw is not substantial. The issue is sizing a decent solar photovoltaic system with the right storage that can handle your radio or perhaps a few other things -- a little oversizing will prove handy. So when I sized mine, I did the calculations for a radio, but the radio turned out NOT to be the driver. It was other things. Long and short of it: if you have a decent sized panel (they can range from 100W to 300W), that will feed a storage battery, which is your power source for anything you conceive of. I oversized my system for one 245 watt panel and one AGM battery (decent sun will charge it in one average day). But that runs a small freezer for a few hours a day (to act as a refrigerator in power-out longer emergencies) to other things that, combined, might run about the same power. Or, I could substitute a laptop (running via the inverter), a few phones, and a few LED lights. So if you choose a single average decent sized panel and an appropriate AGM with someone helping you size them, then the other things get added: a good charge controller (keeps the battery healthy and tells you about it with a display of some sort), a few switches/disconnects, a few fuses (sized for where and what they do to protect equipment or you), disconnects, and some sort of 12V dc-out rig (I am using a rigrunner). But I can tell you, that what I know, which was limited, needed the assistance of that friend who new solar....and could cut through all the too-much-information that is everywhere on the web. Even with his help and my getting started, its been several hours together and several hours separately...and we have a ways to go. In the end, your radio will get powered, but as I said, it probably will not be the driver for system design, unless you really transmit a lot constantly, which is doubtful. You mentioned a generator. Yes, in most cases they are cheaper. The issue is what you are trying to do. We do disaster assistance (comm is just a part) and we were in parts of the Katrina thing -- where gas could not be gotten easily without driving 100 miles during the first two weeks following the eye crossing the coast. But if you are not concerned about such things and you just want power without the grid, a generator IS an easier solution.

Richard. These guys have been in business for decades, have a real store front and sell both locally and on line. I used them for reference when bidding some jobs a number of years ago. I think you can find reliable information and see what products are actually used.